This command parses substrings from an input string in a fashion similar
to the ANSI C sscanf procedure and returns a count of the number of
conversions performed, or -1 if the end of the input string is reached
before any conversions have been performed. String gives the input
to be parsed and format indicates how to parse it, using %
conversion specifiers as in sscanf. Each varName gives the
name of a variable; when a substring is scanned from string that
matches a conversion specifier, the substring is assigned to the
corresponding variable.
If no varName variables are specified, then scan works in an
inline manner, returning the data that would otherwise be stored in the
variables as a list. In the inline case, an empty string is returned when
the end of the input string is reached before any conversions have been
performed.

Scan operates by scanning string and format together.
If the next character in format is a blank or tab then it
matches any number of white space characters in string (including
zero).
Otherwise, if it is not a % character then it
must match the next character of string.
When a % is encountered in format, it indicates
the start of a conversion specifier.
A conversion specifier contains up to four fields after the %:
a XPG3 position specifier (or a * to indicate the converted
value is to be discarded instead of assigned to any variable); a number
indicating a maximum substring width; a size modifier; and a
conversion character.
All of these fields are optional except for the conversion character.
The fields that are present must appear in the order given above.

When scan finds a conversion specifier in format, it
first skips any white-space characters in string (unless the
conversion character is [ or c).
Then it converts the next input characters according to the
conversion specifier and stores the result in the variable given
by the next argument to scan.

If the % is followed by a decimal number and a $, as in
“%2$d”,
then the variable to use is not taken from the next
sequential argument. Instead, it is taken from the argument indicated
by the number, where 1 corresponds to the first varName. If
there are any positional specifiers in format then all of the
specifiers must be positional. Every varName on the argument
list must correspond to exactly one conversion specifier or an error
is generated, or in the inline case, any position can be specified
at most once and the empty positions will be filled in with empty strings.

The size modifier field is used only when scanning a substring into
one of Tcl's integer values. The size modifier field dictates the
integer range acceptable to be stored in a variable, or, for the inline
case, in a position in the result list.
The syntactically valid values for the size modifier are h, L,
l, and ll. The h size modifier value is equivalent
to the absence of a size modifier in the the conversion specifier.
Either one indicates the integer range to be stored is limited to
the same range produced by the int() function of the expr
command. The L size modifier is equivalent to the l size
modifier. Either one indicates the integer range to be stored is
limited to the same range produced by the wide() function of
the expr command. The ll size modifier indicates that
the integer range to be stored is unlimited.

The input substring must be a decimal integer.
The integer value is truncated as required by the size modifier
value, and the corresponding unsigned value for that truncated
range is computed and stored in the variable as a decimal string.
The conversion makes no sense without reference to a truncation range,
so the size modifier ll is not permitted in combination
with conversion character u.

The input substring must be an integer. The base (i.e. decimal, binary,
octal, or hexadecimal) is determined in the same fashion as described in
expr. The integer value is stored in the variable,
truncated as required by the size modifier value.

A single character is read in and its Unicode value is stored in
the variable as an integer value.
Initial white space is not skipped in this case, so the input
substring may be a white-space character.

The input substring must be a floating-point number consisting
of an optional sign, a string of decimal digits possibly
containing a decimal point, and an optional exponent consisting
of an e or E followed by an optional sign and a string of
decimal digits.
It is read in and stored in the variable as a floating-point value.

The input substring consists of one or more characters in chars.
The matching string is stored in the variable.
If the first character between the brackets is a ] then
it is treated as part of chars rather than the closing
bracket for the set.
If chars
contains a sequence of the form a-b then any
character between a and b (inclusive) will match.
If the first or last character between the brackets is a -, then
it is treated as part of chars rather than indicating a range.

The input substring consists of one or more characters not in chars.
The matching string is stored in the variable.
If the character immediately following the ^ is a ] then it is
treated as part of the set rather than the closing bracket for
the set.
If chars
contains a sequence of the form a-b then any
character between a and b (inclusive) will be excluded
from the set.
If the first or last character between the brackets is a -, then
it is treated as part of chars rather than indicating a range value.

No input is consumed from the input string. Instead, the total number
of characters scanned from the input string so far is stored in the variable.

The number of characters read from the input for a conversion is the
largest number that makes sense for that particular conversion (e.g.
as many decimal digits as possible for %d, as
many octal digits as possible for %o, and so on).
The input substring for a given conversion terminates either when a
white-space character is encountered or when the maximum substring
width has been reached, whichever comes first.
If a * is present in the conversion specifier
then no variable is assigned and the next scan argument is not consumed.

The behavior of the scan command is the same as the behavior of
the ANSI C sscanf procedure except for the following differences:

%p conversion specifier is not supported.

For %c conversions a single character value is
converted to a decimal string, which is then assigned to the
corresponding varName;
no substring width may be specified for this conversion.

The h modifier is always ignored and the l and L
modifiers are ignored when converting real values (i.e. type
double is used for the internal representation). The ll
modifier has no sscanf counterpart.

If the end of the input string is reached before any conversions have been
performed and no variables are given, an empty string is returned.